1. Technical Field
The present invention relates to curing method in general, and, in particular, to a method for reducing thin films on low-temperature substrates.
2. Description of Related Art
One approach to making electronic circuits is to print electrical conductors with metallic ink onto a substrate, and the substrate is then heated to sinter the particles of the metallic ink in order to form electrical conducting traces. Generally, most printed metals suitable for electrically conduction need to be heated to a very high temperature, often within a couple hundred degrees centigrade of their melting point, in order to sinter and become conductive.
Two of the most pursued elements for making conductive traces in printed electronics are silver and copper. Silver has two advantages over copper because silver can be heated in air with minimal oxidation and that its oxides, which are comparatively low in conductivity, decompose at relatively low temperatures. These two qualities, coupled with the fact that silver is the most electrically conductive metal often outweigh its high cost when making conductive traces. Thus, even though copper has about 90% of the conductivity of silver and it is usually 50-100 times cheaper on a mass basis, silver inks still dominate the printed electronics market because the additional cost of making and processing copper inks to avoid oxidation is generally higher than the difference in material costs.
It is well-known in the prior art that some metal oxides can be reduced by hydrogen or hydrocarbons at an elevated temperature if they have a positive reduction potential. For example, copper can be first extracted by mixing copper oxide bearing ore with charcoal along with an application of heat. When oxidized copper particles or even pure copper oxide is heated in the presence of a reducer, the oxidized copper particles can sinter to form a conductor.
When making thin film conductors by printing copper particles, a very conductive trace can be formed if the particles are heated to their sintering temperature in an inert or reducing atmosphere. Since the melting point of copper is nearly 1,085° C., the temperature required for sintering dictates that only high temperature substrates such as glass or ceramic can be used. Such high-temperature requirement prevents the usage of inexpensive substrates such as paper or plastic.
Alternatively, if a copper particle film is deposited on a low-temperature substrate, it can be heated to near the substrate's decomposition temperature and then be placed in a reducing atmosphere, but the low temperature dramatically increases the amount of time needed for curing from seconds to minutes or even hours, depending on the thickness of the film and the temperature. At low temperatures, sintering is very limited, and thus the film resistivity becomes high. Furthermore, the need for an inert or reducing atmosphere also dramatically increases processing cost. Thus, it would be desirable to provide an improved method for rapidly reducing metal oxide on low-temperature substrates in ambient atmosphere.